1. Field of the Invention
Example embodiments of the present invention relate to an optical sheet and a display device having the optical sheet. More particularly, example embodiments of the present invention relate to an optical sheet which controls backlight quality in a flat panel display device, and a display device having the optical sheet.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) is a non-emissive type display device, and thus the LCD device requires a light source such as a backlight device to supply an LCD panel of the LCD device with light.
The backlight assembly may be classified as either an edge illumination type backlight assembly or a direct illumination type backlight assembly. In the edge illumination type backlight assembly, a light source such as a cold cathode fluorescent lamp (CCFL) is disposed adjacent to a side edge surface of a light-guide plate (LGP). In the direct illumination type backlight assembly, a plurality of light sources is disposed at the back of the LCD panel.
The backlight assembly includes a plurality of optical sheets that enhance optical characteristics of light exiting from the light source. The optical sheet may include a diffusion sheet that diffuses light, a prism sheet that condenses the diffused light toward a front direction, and a protective sheet that protects the prism sheet, etc.
In order to improve luminance uniformity and general luminance levels, additional diffusion sheets, prism sheets, and/or reflective polarizing sheets, etc., may be used in the display device. However, the additional sheets may cause a cost increase for manufacturing the backlight assembly. As such, it is desirable to improve luminance without additional costs due to the extra sheets. In order to decrease the number of optical sheets the diffusion sheet and the prism sheet are replaced with one optical sheet. In this regard, the one optical sheet should have functions of both the diffusion sheet and the prism sheet.
Moreover, the one optical sheet should not decrease display quality, as may be the case with multiple optical sheets. Particularly, the one optical sheet should satisfy certain visibility conditions, such as luminance and contrast ratio characteristics of the display screen, TCO'03 conditions, etc. (TCO stems from the Swedish language and stands for “Tjänstemännens Centralorganisation,” and the English translation is the “Swedish Confederation of Professional Employees.”) For example, a TCO'03 condition may represent required conditions in which a luminance ratio should be no more than 1.7 at two edges of the display panel when the luminance of the edges of the display panel is observed from a predetermined viewing angle.
As described above, there are various technologies that use one optical sheet having multiple functions. For example, the multifunction optical sheet includes a transparent film, a plurality of prism patterns or a plurality of lenses formed on an upper surface of the transparent film, and a plurality of diffusion beads formed under a lower surface of the transparent film. In another example, the multifunction optical sheet includes a plurality of transparent layers having refractive indexes that are different from each other. In still another example, the multifunction optical sheet includes a diffusing layer having air bubbles formed therein.
However, the multifunction optical sheet developed in accordance with conventional technology may not fully satisfy required display conditions that are achieved by a plurality of optical sheets. Particularly, the conventional multifunction optical sheet may not include a structure for satisfying visibility conditions of a display panel, such as the TCP'03 conditions.
Thus, a need exists for an optical sheet having multiple functions capable of replacing optical sheets used for a display device that satisfies display quality conditions such as luminance, contrast ratio, and TCO'03 conditions required by the display device.